The present invention relates to dehumidifiers and more particularly to a control for de-icing a dehumidifier.
Today""s residential dehumidifier is not designed to operate at temperatures lower than about 60xc2x0 F. to 65xc2x0 F. Some models have a so-called xe2x80x9cde-icerxe2x80x9d. These devices, however, are simply used to shut off the compressor when room temperature falls below the above mentioned temperatures. U.S. Pat. No. 4,745,766 discloses a dehumidifier control system that utilizes a continuously running timer in parallel with an ambient air thermostat to control the compressor. When the ambient air is below a preset temperature, the timer will cycle the compressor on and off while the fan remains running.
U.S. Pat. No. 4,291,542 discloses a dehumidifier which utilizes a: temperature sensor on the evaporator coil which regulates the fan speed and to initiate a defrost cycle. An ambient air temperature sensor is used to bias the preset temperature of the evaporator temperature sensor. The defrost cycle is accomplished by reversing the flow of refrigerant through the system with continuous operation of the compressor and terminating operation of the fan.
U.S. Pat. No. 4,646,529 discloses a refrigeration system which utilizes a sensor to measure evaporator temperature and a sensor to measure ambient air temperature. When either temperature is below a predetermined value for that sensor, a timer accumulates time, and when both the timer has accumulated sufficient time and the evaporator temperature is low, heat will be applied to the evaporator coil to defrost it by means of reversing the flow of refrigerant through the system with the continuous operation of the compressor.
At ambient temperatures below 65xc2x0 F., the evaporating temperature of the refrigerant system falls below 32xc2x0 F. and frost forms on the evaporator coil. In a short period of time the coil is totally blocked and the unit must be defrosted. The evaporator temperature will be relatively stable even when there is light to moderate amounts of frost on the coil. When the gaps between the fins fill with frost, however, the evaporating temperature drops steeply. If the unit continues to run, then the evaporating temperature stabilizes again. Once the evaporator is fully frosted, the frost that is formed is not solid or clear ice. However, if the dehumidifier is operated for an extended period of time, usually over thirty minutes to an hour, then the frost turns into a solid, clear type ice.
It would be an advantage if a relatively simple control were provided which measures a characteristic of the dehumidifier which indicates formation of frost on the evaporator coil, and then terminating operation of the compressor to allow the frost to melt by the continuous operation of the fan drawing ambient air over the coil.
The present invention recognizes that certain characteristics, such as the coil temperature, during the formation of frost on the evaporator coil are predictable and can be used as a basis for defrosting. That is, the evaporator temperature remains stable when there is light to moderate amounts of frost on the coil, but when the gaps between the fins fill with frost, the evaporating temperature drops steeply between the range of 30xc2x0 F. to 10xc2x0 F.
Applicants have determined that a detection of the characteristic of the steep temperature drop, or temperature in this range, can be used to initiate various defrosting strategies.
Also what Applicants recognize is that the initial frost that is formed is not solid or clear ice so that it can be defrosted quickly and efficiently. However, once the frost turns into the solid, clear type ice, this ice is more difficult to melt due to its higher density and takes more time. In such event, the dehumidification effectiveness is reduced.
In one aspect of the invention, a bi-metal temperature switch is selected to operate within the range of the steep temperature drop described above. The switch is used to turn the compressor off (while leaving the unit fan on) and, thus, allow ambient air flow across the evaporator to remove the frost. The bi-metal device is set to shut the compressor off before the evaporator temperature is lower than the area of steep temperature drop, thus preventing the onset for clear ice formation. The bi-metal switch is set to turn the compressor on when the coil temperature has risen above the area of steep temperature drop, as well as above the freezing point, in order to ensure a full melting of the created soft ice or frost.
In another aspect of the invention, a bi-metal temperature switch and a duty cycle timer are combined. The bi-metal switch changes position when the evaporator has entered the steep temperature drop area (indicating frosting conditions) and then enables a timer which cycles the compressor. The timer accumulates the time during which the ice forms, i.e., in which the evaporator temperature is in or below the steep temperature drop area or below 32xc2x0 F. Once the accumulated time reaches a certain value that still guarantees soft ice (typically, but not limited to, 30-60 minutes), the timer switches the compressor off and the ice is defrosted with ambient air by continuous operation of the fan.
In another aspect of the invention an electronic control measures the evaporator temperature with a solid state sensor. Logic in the control then cycles the compressor based upon either the sensed temperature of the evaporator coil, or based upon the rate of downward change of the sensed temperature as described above with respect to the earlier described aspect of the invention.
The invention is not limited to any particular mechanical or electronic arrangement of parts. Electronic measurement, timing and switching can be accomplished in a variety of manners. Further, the control parameters should not be limited to temperature or rate of change of temperature. For example, when measuring the amp draw of the dehumidifier unit, a very distinct and similar behavior can be observed when monitored over time. That is, the amp draw will measurably and quickly decrease when ice is formed over the whole coil. Thus, an amp sensor can be used in lieu of the bi-metal switch or the temperature sensor.